Russia denies claims that its week long campaign of air strikes have mainly hit non-IS targets.

The Syrian Observatory for Human Rights reported the most intense fighting in months in Hama and Idlib provinces. The clashes followed a wave of Russian air strikes in those areas, it said.

They were launched from the Caspian Sea, about 1500 kilometres away.

For its part, the United States has dismissed suggestions that the coalition forces fighting Islamic State militants in Syria might cooperate with Russia.

Instead, the Obama administration made its strongest denunciation yet of Russia's military action in Syria.

Defence secretary Ashton Carter described the intensification of its campaign as a fundamental mistake that was part of a wrong strategy.

The United States was not cooperating with Moscow over the air strikes, he said, beyond discussing technical safety precautions aimed at reducing the chances of a calamitous engagement between Russian and American warplanes.